1. Field of the Invention
The present invention relates to novel ethylene copolymers. More specifically, it relates to ethylene copolymers useful for electric insulating materials.
2. Description of the Prior Art
Olefin polymers, especially, ethylene polymers and propylene polymers are excellent in various properties such as mechanical properties, workability and electrical properties, and are inexpensive and thus economical. Consequently, they are utilized as raw materials for electrical insulating materials and foamed materials, or as finished products such as films, pipes and containers in a variety of field.
Further, there are well known olefin copolymers in which functional groups are introduced into the above-mentioned olefin polymers in order to improve their properties.
For example, polyethylene itself has a small dielectric loss and has a high insulating power, and when crosslinked so as to remarkably improve its thermal resistance, it is employed as a good insulating material. However, for the purpose of utilizing polyethylenes as insulating materials for high-voltage cables and high-voltage equipments, a more improved and more heightened performance is desired for the polyethylenes.
Attempts have been made to improve the insulating power, and a method has been suggested in which an aromatic ring is introduced into an ethylene polymer.
For example,
(1) A method in which an aromatic polymer such as a polystyrene is mixed with a polyethylene or an olefin polymer (Japanese patent publication No. 20717/1963, and Japanese patent Provisional publication Nos. 142651/1975 and 54187/1977).
(2) A method in which a polyethylene is mixed with a block copolymer of styrene and a conjugated diene (Japanese patent Provisional publication No. 41884/1975).
(3) A method in which a graft polymerization is carried out between a polyethylene and styrene (Japanese patent publication No. 18760/1979).
(4) A method in which a polyethylene is impregnated with an electrical insulating oil (Japanese patent Provisional publication No. 33938/1974).
However, even according to any method just described, sufficient improvement of the insulating power cannot be achieved. In addition thereto, as for the method of the paragraph (1) mentioned above, the miscibility of the polyethylene or the polyolefin and the styrene polymer is poor, and in the method of the paragraph (2) above, thermal resistance and extrusion workability are bad. In the case of the method described in the paragraph (3), the previously crosslinked polyethylene is graft-polymerized with styrene in order to improve destructive strength to impulse voltage in a high-temperature range of the polyethylene. In consequence, intricate equipment and processes are required, and there exists the drawback that the destructive strength to the impulse voltage in the low-temperature range is poorer than that of an untreated raw polyethylene. With regard to the method of the paragraph (4), the kneaded electrical insulating oil will be bled out when the polyethylene is used for a long period of time or when environments are changed, so that functional effects of the polyethylene will be poorer than by any other method. Accordingly, the methods are unsatisfactory, and thus the insulating materials are now demanded which are stable for a long period of time and which have an improved performance.